The present invention relates to a method of fabricating production aperture masks used in making thin film circuit components. More particularly, the present invention relates to making apertured deposition masks which are typically used in producing thin film field effect transistors which have very closely spaced source and drain contacts disposed upon a semiconductive channel. The gain of such thin film field effect transistors can be significantly improved when the semiconductive channel is very narrow between the source and drain contacts. It is also known to use interdigitated source and drain finger contacts upon a semiconductive pad to produce an array of electrically parallel thin film field effect transistors.
The conventional fabrication method of making such masks for thin film circuit fabrication consists of a sequence of photoresist delineation, etching, and plating steps. The masks comprise a beryllium-copper core material which is typically about 0.003 to 0.005 inches thick with a strengthening nickel plated layer on either side of the core. The nickel plated layer is typically about 0.0005 inches thick. One side of the mask is termed the relief side of the mask, which is the side closest to the material source during the deposition step. The other side of the mask is termed the defining side of the mask, and the aperture area on the defining side of the mask is typically smaller in area than the aperture opening on the relief side of the mask, and the defining aperture area corresponds to the deposition area on the substrate.
In the conventional fabrication technique the nickel layers are plated over the selectively photoresist coated core portion with a resultant nickel overhang being provided over the photoresist. The nickel layer on the defining side of the mask which remains after the closely spaced apertures are formed is termed a bridge and acts to provide the separation of source-drain transistor contacts during thin film transistor fabrication. The minimum nickel layer bridge which has been found to be achievable repeatably using the conventional fabrication technique provides a nickel bridge that is about 0.0012 inch wide between a source and drain aperture. This fabrication technique thus limits the minimum semiconductor channel width between the deposited source and drain contacts to about the nickel bridge width and limits the gain of the resultant thin film field effect transistor devices.